Caribbean cruise

The sounds on the deck of a cruise ship departing from a southern port on a winter night are the sounds of New Year's Eve. Over the strenuous bubbling hurray-for-us roar of voices, the band is working hard on ''The Saints''; shouts of excited laughter are echoed somewhere in the distance, and a man salutes his friends: ''We made it!'' he exclaims.

Many of the hundreds of merrymakers on the deck are waving frantically to some dozen people standing about on the dock. Pink, yellow, and blue streamers waft slowly against the ship's pure white side, and a sprightly gray-haired lady joyfully flings a peach one to join its fellows. The gay yellow and red pennants that, in the daytime, arch back over the ship from bow to the flying bridge to the stack to the stern, are replaced by small, festive, white lights, celebrating a temporary escape from winter, large decisions, and other hardships for all on board.

Slowly, the Royal Viking Star edges out of the harbor. It's an operation of military precision, navigating such a big ship: 28,000 tons, consuming approximately 464 gallons of fuel per hour at 16.2 knots, with a crew of 370. It almost seems an astonishing amount of effort to take 640 people on a six-day pleasure outing.

A little over a year ago, the Royal Viking Star spent three months in drydock undergoing an unusual expansion: She was cut in half and a new ''middle'' inserted. As Royal Viking's Torbj/orn Soetre said with a deep Norwegian chuckle and a swash of a bushy eyebrow, ''The little baby grew up.''

The ship cost $22 million originally in 1972; the insert, which increases passenger capacity from 500 to 725, cost $100 million. Her sister ship, the Royal Viking Sky, has just undergone the same process; while the Royal Viking Sea goes into drydock in March. The result: the equivalent of a fourth ship, for about $100 million less than it would cost to build one.

You can get around the Caribbean for a lot less than this trip costs. What your extra dollars buy you is space. There is only one seating at dinner, so you sail into the dining room whenever you please; on most other ships you must remember the rights of the person waiting hungrily for your spot at the table. Here you never find yourself contending for the last deck chair; there's usually a sort of forest of them around the pools. Select one, lie down, let the warm air pour over you.

Once in a while though, even the most ship-loving passenger must explore a port or two. Our first stop was St. Thomas, very much on the beaten track for cruise ships. A beautiful and reliable harbor of turquoise water joins the white and pastel town of Charlotte Amalie. By the harbor are old warehouses now picturesquely renovated and divided into expensive little shops selling pearls from Japan, perfume from Paris, etc. The hand embroidered linens seemed a good deal.

St. Thomas is in the path of the trade winds from Portugal - ''That's perhaps why Columbus sailed here,'' said John Bender, our shore excursion manager. We hit St. Thomas on a rare day of rain; our trip out to the underwater marine observatory Coral World was in an open air taxi, and when we arrived there the multicolored and exotic fish were no wetter than our good-humored group.

The history of the Caribbean in the 16th and 17th centuries is a dramatic one: The great powers of the day fought out their rivalries here, and these tiny dots of green land were once ablaze with cannons and burning buildings. The fortifications of the harbor of San Juan, our second stop, are relics of that time. The most spectacular of these, El Morro, built in 1540, proved itself against no less an old sea warrior than Sir Francis Drake. Puerto Rico was then the link between Spain and its Latin American empire; money from Mexico paid for the building of El Morro.

Ms. Velasquez, a friendly young Park Ranger, gave me a tour of the fortress - all dramatic orange stone in the hot sun; we trotted past gundecks, down long slippery stairways and ramps, toured terraces overlooking the gleaming blue harbor. When the US annexed the island in 1898, all the old Spanish texts in the library were simply thrown out, she said. Under the main plaza is a cistern that could contain enough water for the fort for one year.

The winding streets of Old San Juan have a very Spanish quality; many are narrow and cobblestoned and can be traversed only by foot. The air, however, doesn't seem very clean there, and an unaccompanied woman might be made to feel uncomfortable. But there is a lot to see, including several museums, one devoted to Pablo Casals, and an elegant house, Casa Blanca, which once belonged to Ponce de Leon's son-in-law.

The morning spent wandering in Old San Juan was followed by an afternoon in the El Yunque rain forest. El Yunque (white land) is the highest peak of the Luquillo Mountains, which have been protected since they were proclaimed Spanish crown lands in 1876. At the top, we were told, it rains 200 inches a year, making the area a virtual swamp; its trees, too, are stunted from the high wind.

But the lower levels are just benevolently cool and damp and lush with stands of bamboo; our van wound up a rocky hillside coated with moss and adorned with boas of ferns, occasional scarlet impatiens, and tiny waterfalls. Lianas, great woody vines, link the trees. You have to be here early in the morning to catch a glimpse of the almost extinct Puerto Rican parrot.

Our next stop was supposed to be St. Martin, an island half French, half Dutch; from the ship it looked like Hawaii with its sharp green hills. Though we were able to anchor - a very interesting operation in itself - high winds and steep waves made boarding the tender impossible. So that passengers would receive the correct number of islands for their money, our captain substituted Martinique for St. Martin, and the Star, engines throbbing, set her new course.

The Caribbean islands have a high volume of trade but not with each other; instead they tend to deal most with the country with which they share a history and a language. Martinique is a French department, as the traveler can easily deduce from the elegant women in designer jeans and high heels who can be seen on the streets of Fort-de-France.

A pretty black guide took us to a museum at the ruins of St. Pierre, destroyed by the eruption of Mt. Pelee in 1902. ''Everybody here lives by the feesh. We sell it plenty on the is-land,'' she explained, as we passed six men pulling at a net from the beach, which was adorned with green and orange boats lying parallel on the sand. Martinique is extremely pretty but a little more run-down than I had expected: rusting tin roofs and goats nosing among the weeds; it also boasts breadfruit, mango, and guava trees and fountains of wild poinsettias.

It's always nice to come back to the ship, with its spanking blue and white and brass decor, two pools for day, two bands for dancing at night, a continuous supply of really excellent meals (I calculated that there was one half hour during the day when you couldn't get something to eat). Try to aim for a full moon; it's wonderful to stroll on the deck then, trying not to be too smug about the fact that it's snowing up north.

There is no place more peaceful than the deck of a ship at night. On the Royal Viking Star, great globes light your way; you can hear the sound of the engine, perhaps a distant piano, or faint splashing from someone taking a pre-dinner dip in the pool. If you time your walk well, there is the sight of the receding lights of the harbor. It's possible to totally, completely, relax.

Practical details: I wouldn't suggest going alone on this sort of cruise; if you aren't half of a couple, bring a friend. By the way, passengers on this expensive and high-quality line tend to be older and very dignified.

To calculate costs: a deluxe cabin is from $499 to $725 per person per day, an average is $224 to $329; an average double outside is from $199 to $292; and minimum is $121 to $221. The ship can take 725 passengers; the most expensive and least expensive cabins sell first.

Clothes are attractive but casual; for women, white cotton pants for day and black silky formal ones at night make a sensible wardrobe. Both men and women should bring a jacket as the ship is air conditioned at night. Be prepared for several formal evenings.